Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks, cards or tape, and the like. Magnetic tape media remains an economical medium that is particularly useful in storing large amounts of data. For example, magnetic tape cartridges, or large spools of magnetic tape are commonly used to back up large amounts of data for computing centers or businesses. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or laptop computers.
In magnetic tape, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the magnetic tape is often organized along “data tracks,” and read/write heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks. Other types of data storage tape include optical tape, magneto-optic tape, holographic tape, and the like.
As the number of data tracks on data storage tape increases, the data storage capacity of the data storage tape likewise increases. However, as the number of data tracks increases, the tracks usually become narrower and more crowded on the surface of the data storage tape. Accordingly, an increase in the number of data tracks can make positioning of the read/write head relative to a desired data track more challenging. In particular, for proper data storage and recovery, the read/write head must locate each data track, and follow the path of the data track accurately along the surface of the data storage tape. In order to facilitate precise positioning of the read/write head relative to the data tracks on the data storage tape, servo techniques have been developed.
Servo information refers to signals, patterns or other recorded markings on the data storage tape that are used for tracking purposes. In other words, servo information is recorded on the data storage tape to provide reference points relative to the data tracks. A servo controller interprets detected servo information and generates position error signals. The position error signals are used to adjust the lateral position of the read/write head relative to the data tracks so that the read/write head is properly positioned along the data tracks for effective reading and/or writing of the data. A variety of different servo patterns have been developed, including time-based servo patterns, amplitude-based servo patterns, and other types of servo patterns. Time-based servo techniques typically make use of servo marks and time variables, as the servo marks feed past a head at a constant velocity. Amplitude-based servo techniques typically involve the detection of servo signal amplitudes, which enables identification of head positioning relative to the medium.
With some data storage tape, such as magnetic tape, the servo information is often stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. Conventional servo tracks typically hold no data except for information that is useful to the servo controller to identify positioning of a read/write head relative to the surface of the data storage tape. Alternatively, servo information may be interspersed within the data tracks, e.g., at regular intervals.
In any case, the servo information is typically recorded during media fabrication. Then, the servo information is sensed by one or more servo heads during use of the medium in order to pinpoint locations of the data tracks. For example, servo heads may be dedicated heads that read only servo information. Once the servo head locates a particular servo track, one or more data tracks can be located on the medium according to the data track's displacement from the servo track. The servo controller receives detected servo signals from the servo heads, and generates position error signals, which are used to adjust positioning of a read/write head relative to the data tracks.
The ability to properly guide the magnetic tape during media fabrication, servo recording, data recording and data readout can be a limiting factor in achieving improved track densities on magnetic tape, or other data storage tape. For example, the ability to record an increased number of servo tracks on magnetic tape can be limited by the ability to properly guide the magnetic tape during servo writing. Moreover, the ability to increase the density of servo tracks, and thereby allow for increased density of data tracks, can also be limited by tape guiding limitations. Furthermore, the ability to read the servo patterns, or to record and readout data tracks presents similar tape guiding challenges. As track densities continue to progress, these tape guiding challenges will become more significant.